It is becoming increasingly evident that epigenetic mechanisms such as micro (mi) RNAs modify the production of drug receptors, metabolizing enzymes and drug efflux transporters thus affecting therapeutic dose, resistance and toxicity. Altered expression and bioavailability of drug metabolizing enzymes and transporters (DMETs) has been observed in pregnant and lactating women with the potential to affect not only mothers but also child health and development. Mechanistic understanding of this site of pregnancy physiology and how miRNAs targeting DMETs are regulated in these two unique life settings is largely unexplored. MicroRNAs are small, non-coding RNAs that negatively regulate gene expression at the post-transcriptional level by inducing translational inhibition or mRNA degradation. They are encoded by their own genes or within introns and mature through a multistage process, both of which controlled by a variety of exposures not yet deciphered in pregnant women. Under a recently NICHD-awarded 5 R01HD099091-02 we are utilizing large cohorts of Ugandan, Zimbabwean and US women to make inroads in identifying hormonally and microbiome-regulated miRNAs that convey resistance or susceptibility to infection in reproductive age women ? an area of research unexplored to date. In just one year of funding we published results in two original articles and a presentation at the international Conference on Retroviruses and Opportunistic Infections (CROI 2020). The extant metadata catalog includes rich demographic and behavioral factors, infection status, systemic and mucosal levels of innate immunity biomarkers predictive of sexually transmitted infections, dysbiosis and HIV, and systemic levels of endogenous estradiol, progesterone and sex-hormone binding globulin. The current R01 is focused on hormonal contraceptive use and therefore has excluded pregnant and lactating women from the global miRNA transcriptome analysis but we have all metadata available for these women and their longitudinal samples. This response to NICHD NOT-HD-20-003 proposes to leverage existing biospecimens, data and technological resources accumulated during the parent R01 and expand testing to include existing biospecimens from pregnant and lactating women toward the following aims: 1) determine effect of pregnancy and breastfeeding on levels of validated miRNAs regulating DMETs; 2) explore reproductive hormones- and microbiome-driven enrichment for miRNAs targeting known placental drug transporters; 3) identify miRNAs differentially expressed during pregnancy or breastfeeding known to regulate exposure to antiviral and anti-inflammatory drugs, with emphasis on drugs with limited pharmacokinetics studies in these conditions exemplified by experimental therapeutics for COVID-19. The proposed research will generate high-dimension data to become available through the NICHD DASH. It will open the door to novel in silico predictive models of drug safety and availability in pregnant and lactating women, will advance our mechanistic understanding of potential drug-drug interactions, and may identify innovative modifiable factors of drug exposure and novel therapeutic targets in highly vulnerable conditions of mother and child.